1. Field of the Invention
The present invention relates to a material made of magnetic garnet single crystals formed by liquid-phase epitaxial growth (hereinafter referred to as "LPE"), and particularly to a material for magneto-optical elements which is made of magnetic garnet single crystals having a composition of (NdBi).sub.3 Fe.sub.5 O.sub.12.
2. Description of the Related Art
A material made of magnetic garnet single crystals, which has a Faraday effect, is used as an essential material for an optical isolator. At present, the optical communication systems wavelength bands centered at 1310 nm and 1550 nm. In recent years, as a material made of magnetic garnet single crystals used in these wavelength bands, a single crystal film (LPE film) formed on a non-magnetic garnet single crystal substrate by an LPE process has been mainly used. The reason for this is that the LPE process is excellent in terms of mass-production. A typical substrate, used in the above LPE process, is made of (CaGd).sub.3 (ZrMgGa).sub.5 O.sub.12 having a lattice constant &lt;a&gt; of 12.496 .ANG..
Characteristics of a material made of magnetic garnet single crystals are evaluated on the basis of a performance index F (deg/dB) expressed by a relationship of F=.theta..sub.F /.alpha., where .theta..sub.F indicates a Faraday rotation coefficient (deg/cm) and .alpha. indicates an absorption coefficient (dB/cm). The performance of a material made of magnetic garnet single crystals is proportional to the performance index F. The performance index F is increased with an increase in the Faraday rotation coefficient .theta.F and with a decrease in the absorption coefficient .alpha.. In this regard, methods for increasing the Faraday rotation coefficient .theta.F have been proposed, for example in Japanese Patent Laid-Open Publication Nos. 1-246520/1989 and 1-250924/1989, wherein the substitution ratio of Bi in a material made of magnetic garnet single crystals is increased. Moreover, methods of decreasing the absorption coefficient .alpha. by addition of a bivalent or quadrivalent compound or by oxidation and reduction after growth of the crystals have been proposed (Journal of Japan Society of Applied Magnetism, Vol. 10, No. 2, 1986).
Incidentally, in recent years, an optical fiber amplifier capable of amplifying an optical signal as it is, has been actively developed. A praseodymium doped fiber amplifier is used in a communication wavelength band centered at 1310 nm, and an erbium doped fiber amplifier is used in a communication wavelength band centered at 1550 nm. However, so as to practically use each of these amplifiers, an optical isolator for a pumping light source for a stable pumping operation is required. The wavelength of pumping light of the praseodymium doped fiber amplifier is 1017 nm, while the wavelengths of pumping light of the erbium doped fiber amplifier are 980 nm and 1480 nm. Accordingly, an optical isolator usable in each of wavelength bands centered at 980 nm, 1017 nm and 1480 nm is required, and to meet this requirement, a material made of magnetic garnet single crystals suitable for each wavelength band must be developed.
At present, the existing material made of magnetic garnet single crystals for a wavelength band centered at 1550 nm is diverted to a material for a wavelength band centered at 1480 nm, with no problem. On the contrary, the existing material made of magnetic garnet single crystals for a wavelength band centered at 1310 nm cannot be substantially diverted to a material for wavelength bands centered at 980 nm and 1017 nm because it has large optical absorption and thereby the characteristics thereof are deteriorated. The reason why the optical absorption is thus increased is that the absorption of Fe.sup.3+ having a peak near 900 nm is extended to about 1000 nm.